The present disclosure relates generally to compositions containing hydrogen peroxide (H2O2). In particular, compositions stabilizing hydrogen peroxide in alkaline solutions are described.
Hydrogen peroxide is useful in a broad range of applications, for example, as a household cleaner, or in numerous industrial applications, including cleaning integrated circuits, lithographic processing, and coating drilling bits with hard films.
In the high-tech industry, a mixture of H2O2 and Ammonium hydroxide provides an alkaline (pH>7) solution, which can oxidize and remove surface-bound organic contaminant molecules. With regard to integrated circuits, a well-established wafer cleaning method known as the RCA method employs an alkaline H2O2/Ammonium hydroxide mixture, commonly referred to as the standard clean 1 (SC1) formulation. A common problem with the use of SC1 is that new batches of the SC1 solution need to be frequently mixed as the H2O2 decomposes rapidly at temperatures above room temperature, making even short-term storage of the cleaning solution difficult.
Modern integrated chip (IC) design provides another application where stabilized H2O2 would be beneficial. Modern IC design entails fabricating transistors and relevant circuit elements in silicon, which are electrically interconnected by high conductivity metals, such as copper. A multilayer electrical wiring of copper tracks is insulated using materials of low dielectric constant, k, to improve the speed of the ICs.
Modern advanced lithographic methods, such as the dual damascene method developed by IBM (Edelstein), utilize a sacrificial metal layer, the so-called hard mask of Ti or TiN. The sacrificial metal layer protects the underlying low k material (i.e., the material with a low dielectric constant) from plasma processing. In subsequent stages of processing, it is necessary to remove the residual hard mask material without affecting other exposed materials, such as copper, low k dielectric, such as carbon doped silicon dioxide (CDO), or silicon nitrides. Furthermore, plasma processing leads to undesirable polymer residues, which also need to be removed before the next processing step.
The traditional approach to remove hard mask material includes chemical mechanical polishing where a slurry of abrasive material removes the film. However, the chemical mechanical removal processes are not optimal since they can physically damage the inner porous low k dielectric layer. The potential for damage increases as the porosity of the dielectric increases and as the dielectric constant of the material decreases.
One method to remove hard mask material involves selective wet etching. To chemically etch Ti and TiN requires several chemical ingredients that react with Ti and TiN films to create soluble chemical species. In general, selective wet etching requires harsh chemicals that can result in some loss of dielectric and copper, the latter by chemical corrosion. Removing the hard mask material with a stabilized H2O2 formulation would be a superior approach.
A brief summary of references relevant to the above described technical challenges is provided in the paragraphs below.
Several peroxide stabilizers employing colloidal particles are known, such as U.S. Pat. Nos. 2,872,293 and 4,320,102. These colloidal systems, as well as phosphate/phosphonates described in U.S. Pat. No. 4,294,575, act to scavenge metal redox ions such as Fe and Cu that are responsible for peroxide decomposition.
Table 1 illustrates the enhanced decomposition of H2O2 as function of temperature or of Cu+2 ion concentration in absence of colloids to help demonstrate the existing limitations of unstabilized H2O2.
TABLE 1Decomposition of hydrogen peroxide in alkaline medium (pH = 10.5). Effect of a copper salt accelerant.Copper SaltTem-(ionic)pera-ConcentrationtureFirst order KHalf-lifeComponents(Ppm (w/w))(° C.)(min−1)(min)H2O2 (23.5 mM) +none21  (7.5 ±10000 ±NH4OH (3.2 mM)2.5) × 10−53000H2O2 (23.5 mM) + 0 (0)65 0.024 ± 0.003  29 ± 4NH4OH (3.2 mM)+ CuSO4.5H2O10 (2.5)210.0029 ± 0.001 240 ± 70+ CuSO4.5H2O20 (5)21 0.014 ± 0.002  48 ± 6+ CuSO4.5H2O40 (10)21 0.020 ± 0.003  35 ± 5Abbreviations: mM refers to millimolar concentration. ppm here refers to parts per million calculated by dividing the weight of the additive by the weight of the solution and multiplying the result by one million. The numbers in parenthesis correspond to the copper ion concentration in ppm units. K is the rate constant for H2O2 degradation; Half-life refers to the time required to decompose half the amount of H2O2 in solution. The concentration of peroxide was determined spectrophotometrically by using Potassium Titanium oxalate.(Reference: Analyst, 1980, 105, 950-954).
Table 1 demonstrates how an increase in temperature of 44° C. or an increase of copper ion concentration, even at ppm levels, decreases the half-life of peroxide in alkaline medium from 3-4 hours to 30-40 minutes. Colloidal systems can bind to ions like copper to suppress degradation. However using colloidal systems can lead to particle contamination, which can create unwanted defects. Therefore, colloidal systems are avoided in the semiconductor industry.
In an alkaline medium, it has been established that OH− ion, or for that matter, most anions except I−, are not involved in decomposing peroxide; although they are postulated to be present in the kinetics of uncatalyzed peroxide decomposition. Certain cations, such as Magnesium (Mg+2), also hinder peroxide decomposition. However, the precise mechanism of peroxide decomposition is dependent on the composition of the peroxide formulation and is, therefore, not well understood.
With regard to the selective removal of Ti and TiN hard masks from a wafer surface without affecting exposed materials, such other metals (Cu, Al, etc.) and insulators (low K and Ultra-low K dielectrics), the references summarized in the following paragraphs are relevant.
U.S. Pat. No. 8,916,479 describes an etching solution including ammonium hydroxide and hydrogen peroxide. The weight ratio of ammonium hydroxide to hydrogen peroxide is between 1:600 and 1:3,000, between 1:1,000 and 1:3,000, or even between 1:500 and 1:3,000. These are the same components used in SC1/RCA1 mixtures.
US Patent Publication 2016/0130500 describes compositions comprising at least one oxidizing agent, at least one etchant, at least one metal corrosion inhibitor, at least one chelating agent, and at least one solvent. The etchants include common bases, such ammonium, potassium hydroxide, Tetramethylammonium hydroxide, and ammonium fluoride. The oxidizing agent could be H2O2 or Fe based compositions. The solvent may include water or small chain alcohols. Complexing agents include ethylenediaminetetraacetic acid (EDTA), ethylenediaminetetraacetic acid diammonium salt, and (1,2-cyclohexylenedinitrilo)tetraacetic acid (CDTA). Corrosion inhibitors include 5-amino-1,3,4-thiadiazole-2-thiol (ATDT), 2-amino-5-ethyl-1,3,4-thiadiazole, benzotriazole (BTA), 1,2,4-triazole (TAZ), tolyltriazole, 5-methyl-benzotriazole (mBTA), and 5-phenyl-benzotriazole.
US Patent Publication 2017/0110363A1 teaches combining a strong base (KOH) and a strong oxidant (hydrogen peroxide). Under the conditions of high pH, as used in these formulations, using inorganic base results in water-soluble titanates. The use of Zn salt and aminopolymethylene phosphoric acid to prevent damage to metal wiring is described.
Thus, there exists a need for stabilized hydrogen peroxide formulations that improve upon and advance the design of known hydrogen peroxide formulations. Examples of new and useful stabilized hydrogen peroxide formulations relevant to the needs existing in the field are discussed in the Detailed Description section below.
Known references relevant to stabilized alkaline hydrogen peroxide formulations include U.S. Pat. Nos. 2,872,293, 4,320,102, 4,294,575, and 8,916,479; and U.S. patent application publication numbers 20160130500 and 20170110363A1. The complete disclosures of the above patents and patent applications are herein incorporated by reference for all purposes.